Ocean acidification alters the calcareous microstructure of the green macro-alga Halimeda opuntia

Decreases in seawater pH and carbonate saturation state (Omega) following the continuous increase in atmospheric CO2 represent a process termed ocean acidification, which is predicted to become a main threat to marine calcifiers in the near future. Segmented, tropical, marine green macro-algae of the genus Halimeda form a calcareous skeleton that involves biotically initiated and induced calcification processes influenced by cell physiology. As Halimeda is an important habitat provider and major carbonate sediment producer in tropical shallow areas, alterations of these processes due to ocean acidification may cause changes in the skeletal microstructure that have major consequences for the alga and its environment, but related knowledge is scarce. This study used scanning electron microscopy to examine changes of the CaCO3 segment microstructure of Halimedaopuntia specimens that had been exposed to artificially elevated seawater pCO2 of 650 µatm for 45 d. In spite of elevated seawater pCO2, the calcification of needles, located at the former utricle walls, was not reduced as frequent initiation of new needle-shaped crystals was observed. Abundance of the needles was 22 %/µm**2 higher and needle crystal dimensions 14 % longer. However, those needles were 42 % thinner compared with the control treatment. Moreover, lifetime cementation of the segments decreased under elevated seawater pCO2 due to a loss in micro-anhedral carbonate as indicated by significantly thinner calcified rims of central utricles (35-173 % compared with the control treatment). Decreased micro-anhedral carbonate suggests that seawater within the inter-utricular space becomes CaCO3 undersaturated (Omega < 1) during nighttime under conditions of elevated seawater pCO2, thereby favoring CaCO3 dissolution over micro-anhedral carbonate accretion. Less-cemented segments of H. opuntia may impair the environmental success of the alga, its carbonate sediment contribution, and the temporal storage of atmospheric CO2 within Halimeda-derived sediments.

Calcareous green alga Halimeda tolerates ocean acidification conditions at tropical carbon dioxide seeps

We investigated ecological, physiological, and skeletal characteristics of the calcifying green alga Halimeda grown at CO2 seeps (pHtotal ? 7.8) and compared them to those at control reefs with ambient CO2 conditions (pHtotal ? 8.1). Six species of Halimeda were recorded at both the high CO2 and control sites. For the two most abundant species Halimeda digitata and Halimeda opuntia we determined in situ light and dark oxygen fluxes and calcification rates, carbon contents and stable isotope signatures. In both species, rates of calcification in the light increased at the high CO2 site compared to controls (131% and 41%, respectively). In the dark, calcification was not affected by elevated CO2 in H. digitata, whereas it was reduced by 167% in H. opuntia, suggesting nocturnal decalcification. Calculated net calcification of both species was similar between seep and control sites, i.e., the observed increased calcification in light compensated for reduced dark calcification. However, inorganic carbon content increased (22%) in H. digitata and decreased (-8%) in H. opuntia at the seep site compared to controls. Significantly, lighter carbon isotope signatures of H. digitata and H. opuntia phylloids at high CO2 (1.01 per mil [parts per thousand] and 1.94 per mil, respectively) indicate increased photosynthetic uptake of CO2 over HCO3- potentially reducing dissolved inorganic carbon limitation at the seep site. Moreover, H. digitata and H. opuntia specimens transplanted for 14 d from the control to the seep site exhibited similar delta13C signatures as specimens grown there. These results suggest that the Halimeda spp. investigated can acclimatize and will likely still be capable to grow and calcify in inline image conditions exceeding most pessimistic future CO2 projections.

CO2-driven seawater acidification increases photochemical stress in a green alga

Increased CO2 and associated acidification in seawater, known as ocean acidification, decreases calcification of most marine calcifying organisms. However, there is little information available on how marine macroalgae would respond to the chemical changes caused by seawater acidification. We hypothesized that down-regulation of bicarbonate acquisition by algae under increased acidity and CO2 levels would lower the threshold above which photosynthetically active radiation (PAR) becomes excessive. Juveniles of Ulva prolifera derived from zoospores were grown at ambient (390 µatm) and elevated (1000 µatm) CO2 concentrations for 80 days before the hypothesis was tested. Here, the CO2-induced seawater acidification increased the quantum yield under low levels of light, but induced higher nonphotochemical quenching under high light. At the same time, the PAR level at which photosynthesis became saturated was decreased and the photosynthetic affinity for CO2 or inorganic carbon decreased in the high-CO2 grown plants. These findings indicated that ocean acidification, as an environmental stressor, can reduce the threshold above which PAR becomes excessive.

Photosynthetically active radiation during the experiment, and lipid content of sub- and intertidal specimens of the endemic Antarctic red alga, Palmaria decipiens

In coastal waters, Antarctic rhodophytes are exposed to harsh environmental conditions throughout the year, like low water temperatures ranging from -1.8°C to 2°C and high light during the summer season. Photosynthetic performance under these conditions may be affected by slowed down enzymatic reactions and the increased generation of reactive oxygen species. The consequence might be a chronic photoinhibition of photosynthetic primary reactions related to increased fragmentation of the D1 reaction centre protein in photosystem II. It is hypothesized that changes in lipid composition of biomembranes may represent an adaptive trait to maintain D1 turnover in response to temperature variation. The interactive effects of high light and low temperature were studied on an endemic Antarctic red alga, Palmaria decipiens, sampled from two shore levels, intertidal and subtidal, and exposed to mesocosm experiments using two levels of natural solar radiation and two different temperature regimes (2-5°C and 5-10°C). During the experimental period of 23 days, maximum quantum yield of photosynthesis decreased in all treatments, with the intertidal specimens exposed at 5-10°C being most affected. On the pigment level, a decreasing ratio of phycobiliproteins to chlorophyll a was found in all treatments. A pronounced decrease in D1 protein concentration occurred in subtidal specimens exposed at 2-5°C. Marked changes in lipid composition, i.e. the ratio of saturated to unsaturated fatty acids, indicated an effective response of specimens to temperature change. Results provide new insights into mechanisms of stress adaptation in this key species of shallow Antarctic benthic communities.

Ocean acidification alleviates low-temperature effects on growth and photosynthesis of the red alga Neosiphonia harveyi (Rhodophyta)

This study aimed to examine interactive effects between ocean acidification and temperature on the photosynthetic and growth performance of Neosiphonia harveyi. N. harveyi was cultivated at 10 and 17.5 °C at present (~380 µatm), expected future (~800 µatm), and high (~1500 µatm) pCO2. Chlorophyll a fluorescence, net photosynthesis, and growth were measured. The state of the carbon-concentrating mechanism (CCM) was examined by pH-drift experiments (with algae cultivated at 10 °C only) using ethoxyzolamide, an inhibitor of external and internal carbonic anhydrases (exCA and intCA, respectively). Furthermore, the inhibitory effect of acetazolamide (an inhibitor of exCA) and Tris (an inhibitor of the acidification of the diffusive boundary layer) on net photosynthesis was measured at both temperatures. Temperature affected photosynthesis (in terms of photosynthetic efficiency, light saturation point, and net photosynthesis) and growth at present pCO2, but these effects decreased with increasing pCO2. The relevance of the CCM decreased at 10 °C. A pCO2 effect on the CCM could only be shown if intCA and exCA were inhibited. The experiments demonstrate for the first time interactions between ocean acidification and temperature on the performance of a non-calcifying macroalga and show that the effects of low temperature on photosynthesis can be alleviated by increasing pCO2. The findings indicate that the carbon acquisition mediated by exCA and acidification of the diffusive boundary layer decrease at low temperatures but are not affected by the cultivation level of pCO2, whereas the activity of intCA is affected by pCO2. Ecologically, the findings suggest that ocean acidification might affect the biogeographical distribution of N. harveyi.

Acclimatization of the crustose coralline alga Porolithon onkodes to variable pCO2

Ocean acidification (OA) has important implications for the persistence of coral reef ecosystems, due to potentially negative effects on biomineralization. Many coral reefs are dynamic with respect to carbonate chemistry, and experience fluctuations in pCO2 that exceed OA projections for the near future. To understand the influence of dynamic pCO2 on an important reef calcifier, we tested the response of the crustose coralline alga Porolithon onkodes to oscillating pCO2. Individuals were exposed to ambient (400 µatm), high (660 µatm), or variable pCO2 (oscillating between 400/660 µatm) treatments for 14 days. To explore the potential for coralline acclimatization, we collected individuals from low and high pCO2 variability sites (upstream and downstream respectively) on a back reef characterized by unidirectional water flow in Moorea, French Polynesia. We quantified the effects of treatment on algal calcification by measuring the change in buoyant weight, and on algal metabolism by conducting sealed incubations to measure rates of photosynthesis and respiration. Net photosynthesis was higher in the ambient treatment than the variable treatment, regardless of habitat origin, and there was no effect on respiration or gross photosynthesis. Exposure to high pCO2 decreased P. onkodes calcification by >70%, regardless of the original habitat. In the variable treatment, corallines from the high variability habitat calcified 42% more than corallines from the low variability habitat. The significance of the original habitat for the coralline calcification response to variable, high pCO2 indicates that individuals existing in dynamic pCO2 habitats may be acclimatized to OA within the scope of in situ variability. These results highlight the importance of accounting for natural pCO2 variability in OA manipulations, and provide insight into the potential for plasticity in habitat and species-specific responses to changing ocean chemistry.

Seawater carbonate chemistry and calcification of the crustose coralline alga Hydrolithon onkodes in a laboratory experiment

Anthropogenic CO2 emissions have exacerbated two environmental stressors, global climate warming and ocean acidification (OA), that have serious implications for marine ecosystems. Coral reefs are vulnerable to climate change yet few studies have explored the potential for interactive effects of warming temperature and OA on an important coral reef calcifier, crustose coralline algae (CCA). Coralline algae serve many important ecosystem functions on coral reefs and are one of the most sensitive organisms to ocean acidification. We investigated the effects of elevated pCO2 and temperature on calcification of Hydrolithon onkodes, an important species of reef-building coralline algae, and the subsequent effects on susceptibility to grazing by sea urchins. H. onkodes was exposed to a fully factorial combination of pCO2 (420, 530, 830 µatm) and temperature (26, 29 °C) treatments, and calcification was measured by the change in buoyant weight after 21 days of treatment exposure. Temperature and pCO2 had a significant interactive effect on net calcification of H. onkodes that was driven by the increased calcification response to moderately elevated pCO2. We demonstrate that the CCA calcification response was variable and non-linear, and that there was a trend for highest calcification at ambient temperature. H. onkodes then was exposed to grazing by the sea urchin Echinothrix diadema, and grazing was quantified by the change in CCA buoyant weight from grazing trials. E. diadema removed 60% more CaCO3 from H. onkodes grown at high temperature and high pCO2 than at ambient temperature and low pCO2. The increased susceptibility to grazing in the high pCO2 treatment is among the first evidence indicating the potential for cascading effects of OA and temperature on coral reef organisms and their ecological interactions.

CO2 and inorganic nutrient enrichment affect the performance of a calcifying green alga and its noncalcifying epiphyte

Ocean acidification studies in the past decade have greatly improved our knowledge of how calcifying organisms respond to increased surface ocean CO2 levels. It has become evident that, for many organisms, nutrient availability is an important factor that influences their physiological responses and competitive interactions with other species. Therefore, we tested how simulated ocean acidification and eutrophication (nitrate and phosphate enrichment) interact to affect the physiology and ecology of a calcifying chlorophyte macroalga (Halimeda opuntia (L.) J.V. Lamouroux) and its common noncalcifying epiphyte (Dictyota sp.) in a 4-week fully crossed multifactorial experiment. Inorganic nutrient enrichment (+NP) had a strong influence on all responses measured with the exception of net calcification. Elevated CO2 alone significantly decreased electron transport rates of the photosynthetic apparatus and resulted in phosphorus limitation in both species, but had no effect on oxygen production or respiration. The combination of CO2 and +NP significantly increased electron transport rates in both species. While +NP alone stimulated H. opuntia growth rates, Dictyota growth was significantly stimulated by nutrient enrichment only at elevated CO2, which led to the highest biomass ratios of Dictyota to Halimeda. Our results suggest that inorganic nutrient enrichment alone stimulates several aspects of H. opuntia physiology, but nutrient enrichment at a CO2 concentration predicted for the end of the century benefits Dictyota sp. and hinders its calcifying basibiont H. opuntia.